[Tests] Refactor Notifier tests to use real filesystem

[gonzaloriestra]

This PR improves the test suite for the Notifier utility in @shopify/theme by replacing brittle filesystem mocks with real filesystem interactions using temporary directories. This aligns with our goal of making tests more trustworthy and reliable.

Changes:

• Refactored packages/theme/src/cli/utilities/notifier.test.ts:
  • Removed global vi.mock('fs/promises').
  • Wrapped filesystem-dependent tests in inTemporaryDirectory.
  • Added assertions to verify actual file content using readFile.
  • Added assertions to verify atime and mtime updates using stat from node:fs/promises.
  • Improved test cleanup by adding a beforeEach block with vi.restoreAllMocks().
  • Refactored the error handling test to simulate a writeFile failure by creating a directory at the expected file path, avoiding the need for manual filesystem mocks.

Verification:

• Ran pnpm --filter @shopify/theme vitest run src/cli/utilities/notifier.test.ts - Passed.
• Ran all unit tests for @shopify/theme package - Passed.
• Verified package integrity with pnpm nx lint theme, pnpm nx type-check theme, and pnpm knip.

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PR created automatically by Jules for task 14554694789806569917 started by @gonzaloriestra

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